Then why is it necessary to follow this or that belief system to get to something so obvious? Why is it necessary to read up on this school of thought or another when the only thing they can teach you is they have nothing to teach you?
Because there's a difference between intellectual understanding of the Self or non-dualism as a concept, and actually manifesting that "truth" by living it. Or as someone once wrote: Philosophical truths are transmitted through "praxis", not like pieces of eight. You've spent years conditioned to think a certain way, decades building that self-model you call "you". These associations will not be destroyed easily, just by someone saying so.
The Hindu/Vedantic systems emphasize self-enquiry and the importance of having a teacher guide you, which may look like pushing belief systems to outsiders, but it's really a process.
The western mystical tradition is more explicit in how important process/techniques are rather than dogma.
Let's put it like this. The main benefits of self-inquiry are not tied to one school of thought. To observe the change in ones thoughts and senses with equanimity does not require anything from old scriptures.
The reason Hindu/Vedantic systems and other systems of spirituality have extra merit is not in what they teach per se, but in the sense of pleasure we get from their poetic expression, and from feeling connected to other practitioners.
That's not to say schools of spirituality have no merit, they do have merit as a social construct, but in terms of self-exploration all you really need is the basic technique and some time to do it.
There is no need. Note that every person has a different conditioning of the mind and not everyone reacts in the same manner to a philosophical statement. Consider several pieces of firewood, each with a varying degree of dryness. Each of them will react differently when ignited with a spark.
The process of understanding the Self requires a teacher, at least in the beginning stages. There are exceptions to the norm, as always. Ultimately, an individual is best equipped to decide what's best for him. Contemplation on the wise sayings of seers may be suitable for some, whereas studying scriptures under a teacher and practicing austerity might be appealing to others.
How else would a person learn or hear about the Self? It is not a solid thing that can be perceived by the senses. Neither is it as abstract as nothingness. It is one's real nature and the source of the notion "I am". Further, it is that which makes it possible for the mind and senses to function.
You may argue that all this information can be gleaned by reading a book. At best, this will remain in the realm of theoretical knowledge. Only a person who has treaded along this path can offer some insights into what the Self is and how it may be realized. If you don't like to label this person as a teacher/preceptor, feel free to give him any other name you like.
To tell a person that he's the body and mind requires no knowledge. Further, such ignorance is well established in every individual. The knowledge of the Self is something a person has to discover after being introduced to it via some medium.
